Tectonics vs eustasy: The oceanic container and its contents

Sea-level change over Earth's history reflects the interplay of water volume and the ever-shifting architecture of ocean basins. While short-term fluctuations (10 3 –10 5 yr) often trace the advance and retreat of glaciers and ice caps, multi-million-year trends (10 7 –10 9 yr) arise from deep-Earth processes – seafloor spreading, subduction, intraplate deformation, mantle plume upwelling, and the emplacement of large igneous provinces – that remodel basin volume and modulate the ocean's water budget. New geodynamic models now discard the assumption of steady-state mantle regassing and degassing, showing instead that persistent imbalances can lock water into the interior or release it back to the surface, sculpting long-term sea-level trajectories. Recent advances in seismic and tomographic imaging, coupled with high-resolution numerical simulations, have fostered an emerging convergence between geodynamic theory and stratigraphic records of Phanerozoic sea-level curves – particularly for second-order (multi-million years) Meso-Cenozoic variations. These efforts also cast doubt on earlier reconstructions based solely on continental flooding metrics without accounting for evolving hypsometries. Superimposed on these tectonic signals are third-order cycles driven largely by Earth's orbital rhythms: long-period Milankovitch modulations (∼1.2 Myr obliquity, especially common during refrigerations, and ∼2.4 Myr eccentricity, recurrent during warm intervals) leave clear imprints in sequence stratigraphic, deep-sea hiatuses, fossil diversity patterns, and stable-isotopic records. Meanwhile, the capacity of small, hydrous mantle plumes to shuttle water across the core-mantle boundary – and the topographic uplift associated with flood basalt provinces – emerges as an underappreciated influence on regional sea-level anomalies. Despite these advances, reconstructing pre-Cretaceous sea-level history remains hampered by scant constraints on ancient spreading and subduction systems. Addressing these gaps and achieving further advancements demands enhanced temporal resolution and more complete datasets – especially for younger intervals where the oceanic lithosphere is preserved with greater fidelity. Seismic and tomographic surveys in under-sampled regions, such as Africa, South America and West Antarctica, are especially critical. Legacy industry data could help fill key gaps, and broader access to publicly funded datasets is vital. Sea-level change stands as one of the most societally-relevant challenges in geoscience and meeting its demands will require sustained investment in advanced data collection, robust modeling, and collaborative partnerships between academia and industry.